1. Field of the Invention
This invention relates to spinal compression/distraction hooks of the type used in the Harrington.TM. system, wherein one or more rods are used to provide compressive or distractive force between pairs of hooks. The system was originally designed to be used to correct scoliotic deformities occurring in adolescent girls, but is now also used to correct other deformities and to repair or stabilize the spine following spinal fracture.
2. Description of Prior Art
The prior art includes a number of devices for use in correction of spinal deformities. These devices have been used primarily for the correction of lateral deviation of the spinal column, known as scoliosis. The spinal curvature which results from scoliosis is generally defined on the basis of specific reference points. In particular, the extreme upper and lower vertebrae and the most displaced vertebrae are of particular interest. The extreme upper and lower vertebrae are those which are the most inclined relative to the median longitudinal axis of the torso. The two planes within which the extreme upper and lower vertebrae can be found define the scoliotic angle. The most displaced, or apical, vertebra is defined as the vertebra which is the farthest from the median axis of the torso.
It is during surgery that the correction is completed and finalized. For this purpose, a solid rod with anchoring hooks is typically placed in the concavity of the curvature and a threaded rod with hooks is placed on the convexity of the curvature. These rods straighten the spine and maintain the correction until arthrodesis is attained by means of autogenous bone graft. The implants used most often to correct curvature during surgery are known as the Harrington.TM. distraction system and the Harrington.TM. compression system. (Harrington is a trademark of Zimmer, Inc., Warsaw, Ind.).
Another use for spinal rods is correction of kyphotic (hunchback) deformities produced by disease or spinal injury. In this use, either two compression or two distraction rods are used. Correction can be achieved by pushing the lamina of the apical vertebra anteriorly with the rods.
The shape and dimensions of existing spinal hooks (FIG. 1A) were generally designed for use on the above-mentioned treatment of scoliosis in adolescent girls. In this use, the hooks and rod act together like a jack to direct a longitudinal (uniplanar) force to elongate the spine and straighten the curvature. In recent years, however, surgeons have also begun treating fractured spines with the same hooks and rods. The typical patient with a fractured thoracic or lumbar spine is an adult male with thicker bones than the young girls for whom most spinal hooks were designed. Secondly, in treating fractured or dislocated spines, two rods rather than one are used. Accordingly, two hooks must be placed under the lamina of the top and bottom vertebral segment to be fixed with the rods.
All existing spinal hooks known to the inventor have four characteristics which make proper placement and implantation of the hooks unnecessarily difficult in many patients,(see FIG. 1A):
The rod housing or body is square. The corners of the anterior side of the hook body jam into the concavely shaped lamina/spinous process junction 95. This prevents the hook from seating fully and often makes it necessary for the surgeon to cut away bone to make space for the square hook body, using an osteotome.
The space between the hook body and shoe 96 is not wide enough to fit around the lumbar spine lamina for many adult patients.
When existing large hooks are placed on both the right and left lumbar lamina in the treatment of spinal injury, the existing broad and square hook shoes often bump into each other making proper seating of the hooks difficult or impossible. The hooks must then be overlapped at their shoes.
Existing hook designs offer either a broad, blunt shoe edge 97 or a sharp cutting shoe edge. Neither design is optimal for placement of distraction hooks in the thoracic spine. In the thoracic spine hooks wedge between two sides of facet joints 98. The edge of the blunt hook shoes are too thick, making insertion difficult. The sharp hook edge can cut into the bone causing a small fracture which can subsequently propagate resulting in hook dislodgement. The sharp edged hooks can also increase the chance of tearing epidural veins causing bleeding when used under either thoracic or lumbar lamina.
When these traction/distraction hooks are used to correct kyphotic deformity, the force exerted by the hook has a significant force vector in the posterior direction. (arrows in FIGS. 1A and 1B). Existing hooks are designed to exert force in the superior or inferior directions and are, therefore, prone to dislodgement when they are used to apply a force in the posterior direction.
Although postoperative hook dislodgement occurs following treatment of all spinal conditions, this problem is especially frequent when hooks and rods are used to correct kyphotic deformity where approximately 10%-15% of the patients have hooks dislodge postoperatively in the first month. Most fractures and dislocations of the thoracic or lumbar spine result in kyphotic deformity. Spinal hooks and rods are used to correct this deformity by combining distraction with 3-point loading of the spine. In this usage, the rod or more ideally, a rod-sleeve pushes forward over the apex of the kyphotic deformity and the hooks pull backwards or posteriorly. Such a sleeve is described in the inventor's U.S. patent specification Ser. No. 159,396 (June 13, 1980), which is hereby incorporated by reference. All existing hook designs known to the inventor feature a large (greater than 5 mm) hook radius. In almost all hooks for use in the thoracic or lumbar spine, portion 96 of the hook between the body and the tip of the shoe is rounded, usually semicircular with a constant radius. To the contrary, the shape of the undersurface of thoracic lamina 99 is essentially flat or slightly concave. Hence, the sole point of contact between the hook and thoracic lamina is at the very edge of the lamina. Particularly when the hook is used to effect a posteriorly directed force to correct kyphotic deformity, a moment is produced within both the hook and the lamina. The hook is pulling the inferior edge of the lamina posteriorly which causes it to tilt. Likewise, since the lamina is making contact with the curved inner aspect of the hook, the hook tilts in the same direction as the lamina. The curved hook then acts as a skid to encourage outward migration of itself relative to the lamina. When the lamina and hook are tilted and the patient bends forward and rotates slightly the hook can dislodge from under the thoracic lamina. In other words, the circular shape of the inner aspect of the hook shoe actually encourages dislodgement from under the thoracic lamina and facets.
The majority of the length of shoes on existing hooks is part of the hook's interior circular radius and, accordingly diverges from the hook body. As a result, the further the hook is advanced onto the flat underedge of the lamina, the further hook shoe tip 97 projects inwardly (or anteriorly) away from the lamina and into the spinal canal. This can be disadvantageous particularly for the spine-injured patient whose spinal cord 100 may be swollen and whose spinal canal may be already narrowed from fractured vertebral body fragments. Any forward tilting of the hook, as described previously, makes the hook shoe project yet further into the spinal canal.
Generally speaking, all previous hooks of which the inventor is aware were designed to direct only longitudinal (compression or distraction) forces against the spine, as opposed to hooks designed to also transmit forces directed posteriorly at approximately at 90.degree. angle to the longitudinal axis of the spine.
There are Harrington.TM. hooks available which have hook shoes extending beyond the rod-engaging body of the hook, for example Zimmer .TM. No. 1279-01 Leatherman Hook.TM.. These hooks do not, however, exhibit a hook shoe which is angled inwardly with respect to the body or possess a narrow radius between the connecting portion and the shoe.
There are several difficulties frequently encountered with existing hooks holders. These include:
Excessive wobble between the holder and hook.
Impingement of the holder on laminar bone.
An uncomfortable handle.
A handle length similar to that of the rod holder.
Difficulty in aligning the holder.
All existing spinal hooks attach to the hook holder by means of a single pair of holes 101 in the hook in FIG. 1A which articulate with a pair of nipples on the hook holder. Since the nipples are opposite each other, they have the same axis of rotation. Existing hook holders also press against the top of the hook body to limit rotation about the nipples.
Particularly, when treating kyphotic deformity and spinal injury, corrective forces are applied to the spine manually by means of the hook holder attached to the spinal hook. Considerable force is often required for these maneuvers. As a result, the hook holder nipples wear away and the hook holder develops excessive "play." This results in excessive wobble between the hook holder and the hook. Because of the excessive wobble, the hook holder tilts and often bumps into the rod holder during the process of hook/rod engagement. Worse yet, the hook holder not infrequently breaks loose of the hook when corrective forces are being applied to the spine via the hook holder.
Another problem with existing hook holders is that the working end of the instrument is essentially square. This square edge at the lower end of the holder often abuts the junction between the lamina and spinous process bone 95 making it difficult to engage the hook holder with the hook once the hook is implanted under the lamina. Another minor disadvantage with existing hook holders is that there is no way to determine correct placement of the holder on the hook when attempting to articulate the two devices.
In the installation of a spinal hook system when repairing kyphotic deformities, two hook holders and one rod holder are used to grip a pair of distraction hooks and a rod. The unratcheted end of the rod is usually inserted into the inferior hook, after the ratcheted end of the rod has been first inserted into the superior hook. The rod is gripped near the inferior end, causing the rod holder to be close to the inferior hook holder. In inserting the rod into the inferior hook, the inferior hook is pulled posteriorly and a torque is applied to the hook by simultaneously pulling the inferior hook holder's handles superiorly. When the hook holder develops free play, it tends to pivot close enough to the rod holder to interfere with the rod holder at the handles. Since as much as 40 kg force is being applied in this procedure, the interference can raise significant problems for the surgeon and his assistants. There is presently no hook and rod attachment system tool set which avoids the above problem of hook interference.